For example, load analyses of hybrid storage systems (tiered storage systems) including hard disk drives (HDDs) and solid state drives (SSDs) have revealed that nomadic work load spikes emerge in some of the hybrid storage systems.
As used herein, the term “spike” refers to the situation where work loads (also known as loads) emerge intensively on a limited area in a storage. The term “nomadic work load spike” refers to a situation where such spikes occurs intensively for a relatively shorter time (e.g., about one to 10 minutes), and then spikes emerge in a different location (offset).
For eliminating such work loads, in addition to HDDs, hybrid storage systems are provided with an SSD as a cache, for achieving both performance improvement and cost efficiency. The scheme where an SSD is employed as a cache is referred to as the SSD cache scheme.
Examples of SSD caches include Facebook FlashCache and Fusion DirectCache.
Unfortunately, such an SSD cache scheme employs the writeback of the cache. Hence, the SSD cache scheme may cause a problem upon migrating nomadic work load spikes with higher write ratios.
Specifically, once all SSD cache blocks have been consumed, for allocating a new spike and cache block, some cache blocks need to be cleaned. For nomadic work load spikes with higher write ratios, a significant amount of writeback to HDDs occurs.
Typical SSD cache blocks have smaller sizes, e.g., 4 kilobytes (KB), and hence a writeback causes a random access to the HDD, which leads to a significant delay.
Additionally, in the writeback cache scheme, once all cache areas are exhausted, a writeback of a dirty block (a block in the SSD the content, data in which does not match the content in the corresponding block in the HDD) occurs frequently. While nomadic work load spikes with higher write ratios are executed, writeback of dirty blocks frequently occurs, which consumes significant areas that can be used by the user.
For the reasons set forth above, applying a cache SDD to work loads experiencing nomadic work load spikes with higher write ratios is often not so effective as expected.
Furthermore, since SSDs are expensive, making full use of them has been sought for achieving their effective utilization.